The present invention relates to the art of electric arc welding and more particularly to a welding apparatus using two consumable electrodes and the method of welding with tandem consumable electrodes.
The present invention utilizes a high frequency inverter of the transistor switching type for converting a three phase input power source to a load coupling transformer from which the AC output of the inverter may be rectified to create current flow between the electrode and workpiece of a welding operation. Such welders employ a pulse width modulator operated at over 18 kHz for controlling the magnitude of the welding current flowing through the welding operation. These inverters are well known in the art and are generally shown in Blankenship U.S. Pat. No. 5,349,157 and Blankenship U.S. Pat. No. 5,351,175, which patents are incorporated by reference herein as background information. They illustrate a three phase inverter with current controlled by a high frequency pulse width modulator directing current pulses to the output transformer of the inverter. The three phase inverter has a pulse width modulator operated by an error amplifier for controlling the current at the output of the inverter. Output switching networks in arc welders for creating AC welding current from DC terminals are disclosed in Stava U.S. Pat. No. 4,861,965 and Stava U.S. Pat. No. 4,947,021, also incorporated by reference herein. Shutt U.S. Pat. No. 4,246,463 and Fratiello U.S. Pat. No. 5,155,330 show tandem mounted consumable electrodes used for welding the joint between two spaced plates. These patents are also incorporated by reference as background information, since they relate to the field to which the present invention is directed.
The present invention relates to an electric arc welder of the type using two consumable electrodes for welding the joint between two edges of relatively thick plates, which plates may be in the form of the cylindrical ends of two pipe sections being welded together in a pipe welding operation or, in practice, seam welding of a formed plate into a pipe. In such welding procedures, two tandem mounted consumable electrodes are moved in unison along the joint so two layers of molten metal are deposited first from the front electrode and then from the trailing electrode to fill the joint between the two beveled edges of the adjacent edges of the plate forming a pipe. These edges are hereinafter referred to as adjacent plates for global application. The invention is applicable for seam welding of pipe; however, pipe welding will be described in a welding operation where the ends of the pipe sections are welded together. The invention is much broader in application and may be used to butt weld two adjacent heavy plates, such as the plates forming sections of gantries for oil rigs, armor of plates used in ship building or seam welding of pipe. Tandem consumable electrodes deposit large amounts of molten metal and cause the metal to be fused in the joint between the plates at high rates as necessary in welding heavy plates. In the past, a single phase power supply was normally used to produce the welding current for both of the tandem electrodes. The frequencies of the welding operation for the adjacent electrodes were the same causing extreme arc generated interference. Such systems required elaborate connections, for instance a Scott connection that produce an electrical phase shift. The frequencies of the welding procedures were dictated by the line frequency of the single phase input power supply. Thus, the frequencies of the pulsating welding current for the tandem mounted electrodes was generally the same fixed value determined by the 50 Hz or 60 Hz input voltage. The prior art welders using tandem consumable electrodes had generally caused an unbalance in the three phase power system and had welding frequencies controlled by the line frequency of the input voltage. This limitation was extremely detrimental when the high currents of pipe welding were used, which high currents exceed about 200 amperes and were often as high as about 1000-1200 amperes, or more. When tandem mounted electrodes are subject to relatively fixed low frequencies, determined by the line frequency, and conduct extremely high currents, used in pipe welding and welding thick plates, arc interference presents a serious problem requiring complicated connections and shielding. In the prior devices, it was somewhat normal practice for each of the electrodes to be driven by current having the same frequency, such as 50 Hz or 60 Hz. The only way to reduce arc interference was to shift the phase of the line current being directed to each of the tandem electrodes. This procedure was extremely complicated. The process was dictated by the line frequency and the phase shifting did not fully alleviate arc interference. Increasing the frequency of the welding current above 100-200 Hz to reduce interference was not practical in the prior systems. Thus, there is a substantial need for an improved electric arc welder using two or more consumable electrodes which do not have the problem of arc interference and do not utilize high current pulse frequencies.
The present invention overcomes the problems experienced in efforts to provide an electric arc welder that can effectively utilize two consumable electrodes, while reducing or generally eliminating arc interference without requiring high frequencies for the welding current. Although the invention can be used in a DC mode, preferably it involves an electric arc welder that directs AC welding currents to two separate consumable electrodes, which welder can be driven by a single three phase power supply while the welding operation at each electrode is independently controlled. The frequency of the welding current for each of the two electrodes is independently controlled and does not depend upon the input line frequency. By using the present invention, a three phase input voltage is used for a tandem electrode welder. Thus, there is a balanced input power, not an unbalanced single phase as in the prior art. The three phase voltage input has a line frequency of 50 Hz or 60 Hz; however, this frequency does not dictate the frequency of the welding current at each consumable electrode. This welder develops high welding current for heavy plates with the current exceeding about 200 amperes and is normally at least about 1000 amperes. Although two tandem mounted consumable electrodes are used in the preferred embodiment, it is possible to use three or more electrodes in the electric arc welder of the present invention.
In accordance with the invention, the electric arc welder includes a first and second consumable electrode, which electrodes are moved in unison along a welding path between the edges of the two adjacent mutually grounded plates, such as the seam in a pipe manufacturing operation. There are two separate power supplies for passing welding currents between the individual electrodes and the plates forming the grounded workpiece of the welding operation. The power supplies each are constructed to provide low frequency current pulses for the welding operation performed by the individual electrodes. In the preferred embodiment, the power supplies include a three phase voltage input, which input is operated at line frequency, such as 50 Hz or 60 Hz. Since the line frequency is isolated and does not control the output frequency at the individual electrodes, the same three phase voltage source can be used for both power supplies separately controlling the tandem mounted electrodes. The three phase power supply is rectified to convert the input voltage to a DC voltage link and a high frequency switching type inverter converts the DC voltage link to a high frequency AC current. The high frequency switching type inverter is controlled by a pulse width modulator operated at a frequency generally greater than 20 kHz with a duty cycle that is adjusted to control the magnitude of the output current at the electrodes. The high frequency AC current forming the output of the high frequency switching type inverter is directed through an output or load transformer having a secondary winding for driving an output rectifier circuit to provide a positive voltage terminal and a negative voltage terminal. As so far described, each of the individual power supplies for each of the two consumable electrodes is driven by the same three phase source, but creates individual outputs in the form of a positive terminal and a negative terminal. In the specific power supply used in the present invention, the output terminals provide power to a switching network driven at a given low frequency for directing the polarity of the welding current from the terminals of the power supply across one of the electrodes and the two spaced plates forming the workpiece of the welding operation. By using an independently controlled output switching network at the output side of a standard high frequency inverter, the frequency of the welding current, if AC or pulsating DC, is independently controlled by the output switching network so the power supply components before the switching network produce a DC current. The frequency at the output of the power supply that controls the frequency of the current in each electrode is independently controllable and not dependent upon either the input frequency of the power supply or the frequency of the welding current used by the other electrode. This operational isolation frees the tandem electrode electric arc welder to allow individual control of the welding current frequency for each of the separate and distinct consumable electrodes. By this architecture for the electric arc welder, the low frequencies at the two electrodes can be controlled to less than 300 Hz and preferably in the general range of 5-200 Hz. The invention is also operative to provide DC current, either continuous or pulsing in both polarities. Three phase input voltage is used, thus allowing balanced operation of the welder without dictating the frequency of the welding currents.
In accordance with another aspect of the present invention there is provided an electric arc welding apparatus comprising at least a first consumable electrode and a second consumable electrode movable in unison along a welding path between the edges of two adjacent, mutually grounded plates. A first power supply is provided for passing a first welding current at between the first electrode and the plates and a second power supply is provided for passing a second welding current between the second electrode and the plates. Each said power supply includes a three phase voltage input operated at line frequency, a rectifier to convert the input voltage to a DC voltage link and a high frequency switching type inverter converting the DC voltage link to a high frequency AC current, an output rectifier circuit to provide a positive voltage terminal and a negative voltage terminal, and an output switching network operated for directing welding current from the terminals across one of the electrodes and the plates, and a circuit for independently adjusting the output switching networks so the value of the first welding current of the first power supply is different from the second welding current of the second power supply. The switching networks can maintain a continuous DC current of either polarity or an AC current at a low frequency by alternating the switches of the network.
The individual power supplies include circuits for setting the low frequency used by each of the separate electrodes. In accordance with an aspect of the invention, at least one of the power supplies includes means for causing the low frequency of that power supply to vary as a function of time. By using this concept, one of the electrodes is operated at a fixed frequency below about 200 Hz, while the other electrode is operated at a frequency that continues to vary between two extremes, such as 10 Hz to 300 Hz. By varying the low frequency of one power supply between two extremes, while the low frequency of the other power supply is fixed, there is only one frequency at which the two electrodes have identical frequencies. In accordance with another aspect of the present invention, the low frequencies of both electrodes are varied as a function of time, but not in synchronization. Thus, the frequency of the welding current at one electrode sweeps between 10 and 300 Hz. At the same time, the welding current of the other tandem mounted electrode sweeps between 10 and 300 Hz. This sweeping of the electrode welding frequency prevents arc interference and does not require complicated connections. In accordance with still another aspect of the present invention, the inverter between the three phase input voltage and the low frequency welding current is operated at an extremely high frequency, i.e. at least about 20 kHz. This is standard operation for a high frequency switching inverter. The invention uses the inverter stage to isolate the electrically separate input power from the welding parameters for each tandem mounted electrode. The output welding current is pulsating. It can be either low frequency DC pulses or low frequency AC pulses or continuous DC current of either polarity. Indeed, in one example, the low frequency pulses at one electrode are DC pulses, whereas the low frequency pulses at the other electrode are AC pulses. The frequency of the pulses is varied progressively or sweep between 50 and 200 Hz so there is no arc interference caused by beating of the frequencies used in the closely positioned consumable electrodes.
A variety of architectures can be employed for creating the individual power supplies. In accordance with the preferred embodiment, each power supply includes an inverter for converting the AC three phase voltage to a DC current source output having a maximum current of at least 200 amperes with a positive output terminal and a negative output terminal to provide DC energy to an output switching network. The network, in the preferred embodiment of the present invention, includes a first transistor based switch in series with the positive terminal of the power supply, a series inductor segment, one of the electrodes and the spaced plates or workpiece. By closing this series circuit, a pulse of a first polarity is directed to the welding operation. The network also includes a second transistor based switch in series with the negative terminal, a second inductor segment, the same electrode and the plates. Such switching networks are shown in Stava U.S. Pat. No. 4,861,965 and Stava U.S. Pat. No. 4,947,021. Control means ultimately turn the first switch ON and the second switch OFF at a first switch reversing point and turn the second switch ON and the first switch OFF at a second switch reversing point. Consequently, AC high welding current is created with alternate positive and negative current pulses. This switch network creates the low frequency welding current for one of the electrodes. The same architecture is used to create the low frequency welding current of the other electrode. By merely adjusting the control means of the two power supplies, the frequency of the welding current is adjusted. Other electrical architectures can be used for the power supplies so long as they have an input voltage operated at line frequency, an inverter to convert the input voltage to a DC voltage link and a high frequency switching type inverter for converting the DC voltage link to a high frequency AC current that is rectified by a circuit to provide a positive voltage output terminal and a negative voltage output terminal. These terminals are used for directing DC energy to the switching network. Thus, the power supply is driven by a three phase voltage source at a line frequency, but creates the desired low frequency output current. The ability to control the low frequencies of the output currents independent of each other results in an improved tandem electrode electric arc welder.
Another aspect of the present invention involves use of a first and second power supply for driving first and second consumable electrodes moved in unison in the joint between two plates. Each of the power supplies have switch networks for creating low frequency welding currents. By generating a synchronizing signal with a given frequency determined by the rate of synchronizing commands, the power supplies are forced to a given polarity when a synchronizing command, such as a logic 1 or a positive going leading edge, is presented to the switching network of the power supply. By delaying the synchronizing signal at one of the power supplies, the low frequency of that power supply is offset or phase shifted. Consequently, by use of a synchronizing signal and a delay circuit, the low frequencies of the welding currents are out of phase. Using a synchronizing signal, the alternating weld current is forced to a given polarity at the time of a command in the signal. In one embodiment, the synchronizing signal is a positive command and a negative command. When the switching circuit receives a positive command, the weld current is forced to positive polarity. The negative command forces the weld current to a negative polarity. Since the synchronizing signal is delayed at one power supply, the forcing action of the command makes low frequency welding currents which are out of phase. This is done without a Scott connection. As another aspect, the synchronizing command starts a weld current oscillating at a given frequency created at the power supply. This given frequency produces alternations between synchronizing commands or between a positive command or a negative command.
The primary object of the present invention is the provision of an improved tandem electrode electric arc welder, which welder can be used for seam welding pipe from a rolled plate and similar welding operations employing pulsating high welding currents without arc interference.
Yet another object of the present invention is the provision of an electric arc welder, as defined above, which welder includes separately driven consumable electrodes, each of which is driven by a high welding current having a low frequency with the frequencies being adjusted or varied on a time basis or is operated at a given DC polarity.
Still a further object of the present invention is the provision of an electric arc welder, as defined above, which electric arc welder utilizes a three phase input voltage allowing balancing of the input power for the welding operation. Such input power balancing is extremely important when using extremely high currents, as needed for welding the seam of a pipe.
Still a further object of the present invention is the provision of an improved electric arc welder, as defined above, which electric arc welder can use tandem consumable electrodes without unbalancing the power source or creating undue arc interference.
These and other objects and advantages will become apparent from the following description taken together with the accompanying drawings.